1. Technical Field
The present invention relates to a position detection system that optically detects a position of a detection target and a projection display system.
2. Related Art
In the past, position detection systems like touch panels have been used for electronic devices with display screen such as cellular phones, car navigation systems, automated ticket vending machines, and automated teller machines. The position detection system is provided at the visual recognition side of the display screen and detects a position of a detection target like a pen or a finger by placing the detection target on the display screen while referring to an image on the display screen. For example, they are disclosed in Patent Document 1 (U.S. Pat. No. 5,666,037) and Patent Document 2 (U.S. Pat. No. 6,927,384).
The position detection systems disclosed in Patent Document 1 and Patent Document 2 are optical systems, and each has a detection space set on a reference surface of a body surface and includes a light source and a light detection unit. When a detection target is placed in an arbitrary position within the detection space, the light of the light source is reflected on the surface of the detection target and a part of the reflected light is received by the light detection unit, and thereby, the position of the detection target is detected.
However, in the position detection systems disclosed in Patent Document 1 and Patent Document 2, if two detection targets are placed at the same time within the detection space, the light detection unit may not respectively distinguish the reflected lights of the two detection targets, but may detect the intermediate position between the two detection targets. Accordingly, there has been a problem that it may be impossible to detect the respective positions of the two detection targets.